Ink concentration as measured by an ink cell, used in optical concentration measurement devices, affects the ink jet printing process. As the ink concentration varies, the optimal parameters used to control the ink jet printing process change as a function of the ink concentration. Currently, a variety of methods exist for calibrating optical concentration assemblies in order to determine ink concentration effects. For example, a typical optical concentration device includes a light source which directs a path of light toward an ink cell. A photo diode is situated such that the ink cell is between the light source and the photo diode. The light from the light source is first measured with the ink cell in place, such that the light received by the photo diode is filtered through the ink cell. The ink cell is then moved out of the light path, so the light from the light source shines directly on the photo diode. A comparison between the light received through the ink cell and directly with the ink cell removed is made to calibrate the optical concentration device. However, it is a time consuming process to have to continually move the ink cell in order to update calibrations.
Alternatively, then, calibration can be accomplished manually, without moving the ink cell. In this instance, a known reference, such as ink quality in the ink cell, is used to manually calibrate a light source, such as an LED, output to some nominal value. The LED light is directed through the ink of known quality toward a photo diode, and the light output to the photo diode is used to determine the LED output. This method, however, ignores the effects of temperature drift. Since absorption, which increases exponentially with concentration, varies with ink temperature, it is important to be able to compensate for temperature drift so that the ink concentration can be controlled. Additionally, ink pressure affects the dimensions of relatively inexpensive ink cells, affecting the ink concentration measurement. Unfortunately, ink cells which are less susceptible to pressure changes are also more expensive. Finally, in known methods of ink concentration measurements, the ink cell is required to be placed very close to the measuring circuitry to avoid noise problems.
It is seen then that there exists a need for an ink concentration measuring circuit which accounts for variables which affect ink concentration, including ink temperature and ink pressure.